NAME
shmget —
get shared memory area
identifier
SYNOPSIS
#include
<sys/shm.h>
int
shmget(key_t key,
size_t size, int shmflg);
DESCRIPTION
shmget()
returns the shared memory identifier associated with the key
key.
A shared memory segment is created if either key is equal to IPC_PRIVATE, or key does not have a shared memory segment identifier associated with it, and the IPC_CREAT bit is set in shmflg.
If a new shared memory segment is created, the data structure associated with it (the shmid_ds structure, see shmctl(2)) is initialized as follows:
- shm_perm.cuid and shm_perm.uid are set to the effective uid of the calling process.
- shm_perm.gid and shm_perm.cgid are set to the effective gid of the calling process.
- shm_perm.mode is set to the lower 9 bits of shmflg.
- shm_lpid, shm_nattch, shm_atime, and shm_dtime are set to 0
- shm_ctime is set to the current time.
- shm_segsz is set to the value of size.
- The ftok(3) function may be used to generate a key from a pathname.
RETURN VALUES
Upon successful completion a positive shared memory segment identifier is returned. Otherwise, -1 is returned and the global variable errno is set to indicate the error.
ERRORS
The shmget() system call will fail if:
- [
EACCES] - A shared memory segment is already associated with key and the caller has no permission to access it.
- [
EEXIST] - Both IPC_CREAT and IPC_EXCL are set in shmflg, and a shared memory segment is already associated with key.
- [
EINVAL] - No shared memory segment is to be created, and a shared memory segment exists for key, but the size of the segment associated with it is less than size, which is non-zero.
- [
ENOENT] - IPC_CREAT was not set in shmflg and no shared memory segment associated with key was found.
- [
ENOMEM] - There is not enough memory left to created a shared memory segment of the requested size.
- [
ENOSPC] - A new shared memory identifier could not be created because the system limit for the number of shared memory identifiers has been reached.
LEGACY SYNOPSIS
#include
<sys/types.h> #include
<sys/ipc.h> #include
<sys/shm.h>
int
shmget(key_t
key, int size, int
shmflg);;
All of these include files are necessary. The type of size has changed.